Improve: file not found in alpine image

[tikinang]

add an entry to docs about errors executing binaries on alpine - probably to general FAQ page (feel free to suggest a different location)

in most cases it is caused by missing glibc in alpine linux. easiest solution is to use ubuntu if possible. otherwise installing musl libs can help.
example of the error:

tegola-runtime-2-7e:/var/www$ ./tegola 
bash: ./tegola: cannot execute: required file not found